There is a need in the art for simple, sensitive, and robust bioassays to detect costimulatory activity of various proteins known to costimulate T cells, such as, for example, B7-1 (also known as CD80), B7-2 (also known as CD86), and B7-Related Protein 1 (B7RP1, also known as ICOS Ligand (ICOSL or LICOS), B7 Homolog 2 (B7H2), and GL50) on T cells. Beyond detecting functional costimulatory proteins in samples, such bioassays can be modified to detect therapeutic drugs that inhibit these costimulatory proteins, as well as neutralizing anti-drug antibodies that may develop in dosed subjects and may block the inhibitory activity of such drugs. For example, inhibitors of B7RP1 have potential use as drugs to treat, for example, systemic lupus erythematosus. See, e.g., Kow and Mak (2013), Costimulatory Pathways: Physiology and Potential Therapeutic Manipulation in Systemic Lupus Erythematosus, Clinical and Developmental Immunology, Article ID 245928 [online][retrieved Apr. 9, 2014 from Hindawi Publishing Corporation available at http://dx.doi.org/10.1155/2013/245928]. Simple, sensitive, and robust bioassays to detect such therapeutics, as well neutralizing anti-drug antibodies that may develop in patients, are needed.